Relationships between brain anatomy, brain chemistry and behavior/cognition will be characterized among children with autism in comparison to children with mental retardation and normal siblings of children from the two clinical groups. These relationships will be compared at 2 time points to assess underlying brain developmental processes hypothesized to be operative among children with autism. Specifically, in reference to the well-replicated imaging findings of increased brain volum in autism, if brain hypertrophy reflects hyperplasia arising from over-proliferation of neurons and/or disruption of apoptosis during the pre or perinatal periods, then group differences in brain anatomy/brain chemistry are expected to remain stable across the two time points. If, on the other hand, structure abnormalities reflect synaptic pruning abnormalities with onset during the preschool period, or gliosis, emergance or amplification of group differences is expected at the second time point. For samples of 60 children with autism, 40 children with mental retardation, and 20 age-matched normal siblings aged 3-4 years old, measures of brain anatomy from 2-D and 3-D and 3-D MRI, regional brain chemistry from 2-D proton echo-planar spectroscopic imaging (PEPSI) and behavior/cognition will be obtained. Children with autism and those with mental retardation will be followed longitudinally and these measures reassessed at 6-7 years of age. An additional group of normal siblings will be assessed at ages 6-7 to provide normative data. As part of this proposal, addditional normative brain volumetric data will be made available by Dr. Jay Giedd at the NIMH which will be used also to assess measurement reliability between sites. Morphometric analysis using intensity-based segmentation techniques, subregion segmentation and 3-D surface rendering techniques will be used to characterize subtle differences in brain anatomy. PEPSI will be used to examine regionally-specific differences in brain chemistry; for brain anatomical regions exhibiting structural abnormalities, brain chemistry patterns of N-acetyl asparatate (NIAA), choline, and lactate levels will help to distinguish whether such abnormalities will reflect neuronal as opposed to glial processes. Additionally, patterns of NAA change will help to distinguish abnormal synaptic pruning developmental processes. We predict that brain chemical abnormalities will map both to corresponding brain structural abnormalities and impairments on neuropsychological tasks assessing those brain regions. PEPSI measurements in combination with MRI is expected to be more sensitive and specific for demarcating subpopulations of children with autism than MRI detection of anatomical differences alone.